The present invention relates in general to night vision systems, or so-called starlight scopes, but, more specifically, to a novel apparatus essentially comprising an inverter image tube coupled at its output to a microchannel plate wafer tube for image intensification which combination permits modular construction and economical external photocathode processing techniques without seriously impairing the degree of information extraction.
Typical levels of illumination for which such systems are designed are exceedingly low, as of 10.sup.-2 to 10.sup.-5 foot candles, depending on the moon, cloud cover, etc. As a consequence, the visual gain of such devices must be high since the human eye should be put in the position to utilize cone rather than rod viewing for reason of the higher acuity associated with the former. Further, the quantum efficiency of the first sensing element should be as high as possible so as to optimize the degree of information extraction and minimize the deteriorating effects of quantum fluctuation noise, that is, noise which results from the physical phenomena of unequal numbers of photons being emitted from an emitting body during successive equal time intervals. Additionally, the resolution (or high spatial frequency cut-off point of the modulation transfer function) in terms of line pairs per millimeter (Lp/mm) should be adequate to permit target recognition at the desired range and field of view of the instrument and, lastly, the weight, size and power requirements of the system should be small so as to assure portability.
To meet these prerequisites and the necessary objectives of usefulness and reliability under the above-described condition of low level illumination, any proposed system should have an overall luminous gain of 50,000 (2870.degree. K light in, P-20 phosphor light out) with a resolution of around 30 Lp/mm. The peak quantum efficiency of the first energy converter, typically an S-20 photocathode, may be around 15 to 20%.
Such performance requires, at the present time, three cascaded single-stage visible light inverter image tubes, the input of each stage of which is comprised of a fiber optic component serving as substrate of an S-20 photocathode and the output of which is comprised of a fiber optical component overlaid with P-20 phosphors.
The three stage assemblies perform reasonably well; however, they are far too large, too heavy and too costly. As a result, their general use has been limited to use on hand-carried instruments and then by only a limited number of persons who need and would otherwise use them.
Various proposals have been submitted for reducing the size and weight of night vision systems for application to, such as, for example, individual goggles.
Of the proposals, two look most promising and utilize the recently developed microchannel plates and microchannel plate wafer tubes. In one, a microchannel plate inverter tube, a microchannel plate is inserted in proximity focus within the vacuum envelope of a conventional inverter image tube between the electron focusing electrodes and the phosphor output screen. In another, a microchannel plate wafer tube is coupled at its output to a fiber optic image inverter, called a twister.
Due to device noise believed to be due to geometric and electrical imperfections between channels as well as groups of channels and a high residual gas content in the microchannel plate, high mode performance of the proposed devices necessary for a visual gain of 50,000 has been disappointing.
While the first of the proposals, namely, image inversion by an electron optical system is preferred over the use of a twister to reduce optical losses and distortions, use of the former does lead to complications by virtue of the resulting parabaloidally shaped image plane. Thus, microchannel plate inverter tubes have been found to suffer from an effective radial increase in channel length-to-diameter ratio as the parabaloidally curved electron optical image plane matched to a flat microchannel plate entrance plane yields paraxial first impact points shifted along the channel axis toward the channel exit. The net effect is a paraxially lower microchannel plate electron multiplication gain.